The invention relates generally to the coking of coal.
The thermal pretreatment of coking coal, that is, the preheating of coking coal prior to the introduction thereof into coke ovens, is today widely used in coking practice. The most important coal mining nations utilize this process step in the production of coke for the blast furnace in order to improve the utilization of the coking coal. Thus, by using the thermal pretreatment technique, it becomes possible to increase the proportion, in the starting coal mixture, of coal having poor coking characteristics. Furthermore, the quality of the coke, and especially the resistance to abrasion, is improved by virtue of the thermal pretreatment. In addition, the thermal pretreatment technique provides the advantage that the throughput of coke ovens have horizontal coking chambers may be increased.
Although the thermal pretreatment, which involves heating the coal to temperatures of 130.degree. to 190.degree. C and, advantageously, to temperatures of 150.degree. to 180.degree. C, does not, of itself, cause any technical problems, the handling of the preheated coal, and particularly the feeding of the coal into horizontal coking chambers, poses great difficulties. The reason resides in that only very poor control is possible over that portion of the coal which is in finely divided form, that is, over the fine coal dust. A typical analysis in the particle size for thermally preheated coal might appear as follows:
______________________________________ Coal particles having a size less than 2 millimeters 75-80 percent Coal particles having a size less than 0.5 millimeters 35-50 percent Coal particles having a size less than 100 microns about 18 percent ______________________________________
The current regulations in effect for environmental protection Thus make it necessary to transport the preheated coal from the preheating installation to the horizontal coking chambers via closed conduits and/or containers and to then charge the coal directly into the coking chambers from the conduits and/or containers.
When the preheated coal is charged into a hot coking chamber, there immediately occurs a vigorous degassing of those coal particles which come into contact with the hot stone surfaces of the chamber. These surfaces have temperatures of about 1000.degree. to 1200.degree. C. The crude gases produced in this manner flow through the loose coal charge which has already been fed into the coking chamber and entrain fine coal dust from this charge. Thereafter, the crude gases traverse the coal stream which is in the process of being introduced into the coking chamber and are again loaded with fine coal dust. Thus, crude gas which is highly loaded with coal dust leaves the coking chamber via the uptake conduit during charging of the coal into the chamber. The loaded crude gas escapes into a collecting means. This so-called "carry over", together with the tar which likewise escapes into the collecting means in the form of vapor and which is deposited by means of water, forms a highly viscous mass which is difficult to handle. Moreover, a portion of the finely divided coal dust is carried into the tar separator along with the condensate which flows off from the collecting means and, as a result, difficulties arise during the separation of the tar and the water due to the formation of an emulsion. Finally, the high water and solids content of the crude tar leads to substantial problems in the further processing of the tar in distillation installations.
It has already been attempted several times to reduce the escape of dust, that is, the "carry over", which arises during the charging of coal into coking chambers by binding the finest coal particles to coarser coal particles. In particular, the addition of mineral oil fractions has been proposed in order to effect an agglomeration of this type. This measure, which is relatively simple in itself and which has proved itself in the feeding of moist coal into coking chambers, gives rise to considerable difficulties when preheated coal is used. Thus, when an oil which readily distributes itself over the surface of the coal and which has a low to medium boiling point is used, the oil evaporates before it can become effective for the charging operation. On the other hand, if a heavy fuel oil is selected, uniform distribution of the oil over the total surface of the coal can hardly be achieved inasmuch as the tendency of the fuel oil to wet the preheated coal is low. Moreover, in this case also the oil evaporates partially or completely from the surface of the coal since the oil already tends to evaporate at substantially lower temperatures than would be expected on the basis of the vaporization characteristics of the heavy oil. Consequently, the binding effect of the heavy oil is dissipated to a large extent within a very short period of time.